This invention relates to improvements in a new process for producing electric cables developed by the present inventors.
The new process referred to above is a novel process for producing electric cables having an insulating layer of a cured polymeric material by using a horizontal forming and curing zone and is operable at higher speeds than the conventional V.C.V. process, the cured polymeric insulating layer being free from voids.
According to the new process, a curable polymeric material supplied onto a conductor by an extruder or the like is formed and hot-cured by a long-land die, which is from about 1 to 50 m or more in length. To the inner surface of this die, a specific forming coagent is continuously applied, and the hot-cured insulating layer formed on the conductor is then cooled with a high-pressure cooling fluid in a cooling zone attached to the exit of the long-land die. Usable as the forming coagent in this process are those coagents described in the pending application of M. Fuwa et al, Ser. No. 212,049, filed Dec. 27, 1971, now U.S. Pat. No. 3,928,525, issued Dec. 23, 1975 and assigned to the applicants' assignees.
Although the above described new process for producing cables is satisfactory in the case of producing thin wall insulating cables or cured rubber insulating cables, serious difficulties, such as roughening of the surface of the resultant cables or lowering of the break-down voltage thereof, tend to occur when thick wall, cured-polyolefin insulting cables of voltage rating of 154 KV or 275 KV are to be produced with the use of a curable polymeric material such as polyolefin containing a curing agent, and in some cases the production thereof must be interrupted because of constantly increasing severity of the roughness of the surface of the cable.
As a result of intense studies directed toward clarifying the reason for the above described difficulties, we have found that the melted uncured polyolefin undergoes a viscous flow, which is sharply different from rubber-like compositions undergoing a plug flow, and that when the thick wall insulating cables are produced, the thick layer of polyolefin flowing through the long-land die tends to undergo an abrupt change in direction within a portion of the die ranging from the tapered portion to the land portion. For these reasons, the flowing speed of a part of the flowing layer of polyolefin is slackened along the inner surface at the entrance of the land portion. The slackened layer of the polyolefin is scorched in a very short period and the inner surface of the land portion is encrusted with scorched polyolefin. The scorched polyolefin destroys the molding ability of the land portion, and the outer surface of the cable thus produced is caused to be damaged by the scorched polyolefin on the land portion, and the breakdown voltage of the cable is thereby lowered.
Frequently, the scored polyolefine hampers and further slackens the flow of polyolefin layer near the inner surface of the land portion, the scorching of polyolefin thereby being accelerated, and the production of the cable is ultimately interrupted.
The creation of the scorched polyolefin on the inner surface of land portion depends much on the position at which the forming coagent is introduced into the inner surface of the die. In the previously described new process, the forming coagent has been supplied to the land portion of the long-land die. In this improved process, however, the forming coagent is supplied to the tapered portion of the long-land die, whereby the slackening in the flow and scorching of a part of the polyolefin are thereby prevented, and large-sized cables insulated with a thick wall of cured polyolefin and of high quality and high breakdown voltage can be produced continuously for a long period of time.